1. Field of the Invention
This invention relates to diamond and diamond-like materials having photochemically modified surfaces, and a process for photochemically modifying the surface of diamond and diamond-like materials.
2. Description of the Related Art
Diamond is a metastable phase of carbon possessing remarkable mechanical, thermal, chemical, optical, and electrical properties. Not only is it the hardest material known, it is remarkably chemically inert. It has a low coefficient of friction and a high thermal conductivity. Diamond is optically transparent, and its electronic properties as measured by the Johnson or Keyes Figures of Merit are superior to both silicon and the III-V compound semiconductors. These properties make diamond an excellent candidate for use in wear (coating machine tools), tribological (bearings), optical (radomes and laser windows), and electronic (packaging and semiconductor) applications. Problems have been encountered in the control of the interface between diamond and other materials as a consequence of the very inertness of diamond and the difficulties encountered in modification of its surface.
An effective, reproducible method to introduce functionality into the diamond surface has been long sought. Such a method would allow standard organic reactions to be used to convert the introduced functionality into a diverse array of attached organic and organometallic functional groups, with the ability to customize the diamond material for diverse end-use applications.
The advantages to be gained by developing a versatile approach for diamond surface modification are many. Applications that are likely to be immediately amenable to a surface modification approach include improvement in bearing surfaces and adhesion of diamond to a wide variety of materials including metals, insulators, and semiconductors. Modification of the diamond surface to include polar functional groups such as amines or carboxylic acids will permit bonding of polymers, glasses, and window materials. Tribological failures of interacting component surfaces can be minimized by lowering friction and wear. In the case of diamond, it has been shown that the surface structure has a dramatic effect on friction forces generated when a metal contacts the diamond.
The advantageous thermal and electronic properties of diamond lead to the highest figures of merit for any semiconductor material for applications in high power, high speed, high temperature and high frequency applications. Progress in understanding the growth of diamond films has been significant. At the present time, work is underway in many venues directed to understanding and controlling nucleation and growth so as to obtain monocrystalline films on non-diamond substrates. Once usable films are obtained, device development will require that processing techniques suitable for diamond be devised. Because of diamond's extreme chemical resistance, techniques that have been successful for silicon processing are unlikely to be directly transferable to diamond processing. Examples of processing steps that will be affected by the chemically resistant properties of the diamond surface are photolithography, etching, deposition of insulating layers, and preparation of ohmic and rectifying metallic contacts.
Surface modification promises to give microelectronic device processors a chemical "handle" onto the diamond surface, under conditions significantly easier to implement than the current state-of-the-art approaches for bonding to diamond surfaces, that is, carbon or argon ion bombardment and deposition of an adhesion-enhancing metal (e.g. titanium) followed by a high-temperature anneal.
Surface modification also promises to enable the development of useful new diamond-based materials with interesting properties, produced by ordinary chemical reaction with the initial functionalized surface. Diamond surfaces with introduced functionality will have the capability to be derivatized in virtually any way imaginable, using adjustments of standard organic chemical reactions.
In addition, the development of a controlled diamond surface modification technique would allow the use of photolithographic techniques to produce patterned surfaces. Such photolithographic techniques have been well-developed for silicon and, if the lack of reactivity of the diamond surface were overcome, could be used in many semiconductor processing steps such as formation of contacts, optionally self-aligned, and area-selective chemical-vapor deposition, and other juncture and composite structure applications.
A technique for controlled chemical modification of the diamond or diamond-like surface, by which is meant a technique that could be performed reproducibly under relatively mild conditions and by which the percent of reaction could be selected and reproducibly controlled by reaction time or conditions or reagent concentrations, has proven elusive. While the surfaces of most materials are quite reactive, the diamond surface is relatively unique in its surface chemistry and is generally inert to most common chemical reagents. This is apparently due to the fact that most reaction mechanisms for diamond surface modification are precluded by two major factors: (1) the steric bulk of the diamond surface constrains the attack geometry for an incoming reagent to a near surface-normal vector, and (2) the sp.sup.3 -hybridized carbons of the diamond bulk and the C--H groups presented by the unreconstructed diamond surface are inherently low in reactivity; they offer no useful molecular orbitals for attack by either nucleophiles or electrophiles.
There are in general three exceptions to the chemical inertness of diamond. Diamond is metastable to graphite, the most common allotrope of carbon, and a phase transformation from diamond to graphite can be caused and then the graphite used as the basis for further reactions. The phase transformation to graphite has been effected in many ways: simple heating of diamond, ion bombardment, and by irradiation with high energy or high power laser irradiation. Secondly, the diamond surface may be oxidized. Heating in the presence of O.sub.2 graphitizes the diamond surface prior to any further oxidation. Oxidative surface modification may also be accomplished by an etching procedure employing molten alkali nitrates. This technique is hazardous to personnel and is not especially safe or controllable. Thirdly, direct reaction of diamond with a gas-phase reagent under conditions of free-radical reaction can lead to modification of the diamond surface by radical reactions. The radicals which are known to react directly with diamond include hydrogen atoms, fluorine atoms, and chlorine atoms. The diamond surface is unreactive to the molecular species H.sub.2, F.sub.2, and Cl.sub.2. Although molecular H.sub.2 and Cl.sub.2 are often the reagents used, the reaction conditions required are such that the atomic species are produced and are either very vigorous and corrosive (e.g., Cl.sub.2 /450.degree. C.), unsuitable for implementation on a production scale (e.g., fluorine-atom molecular beams), or have proven irreproducible.
All of these approaches for modifying the diamond surface require aggressive conditions, and the extent of surface reaction cannot be readily controlled. Conversion to graphite will cause the loss of many of the advantages offered by diamond. None of these approaches provides a useful technique for controlled chemical modification of diamond surfaces, that could be performed reproducibly under relatively mild conditions and by which the percent of reaction could be selected and reproducibly controlled by reaction time or conditions or reagent concentrations.
Such a controlled surface modification method would introduce a functional group onto the diamond surface which could be reacted via standard organic reactions, e.g., alkylation, acylation, etc., to convert the introduced functionality into a diverse array of attached organic and organometallic functional groups. Amine groups would be highly useful as the introduced functional groups, since they can be reacted to give a diverse array of products. The amine-terminated diamond surfaces could therefore be derivatized in numerous ways useful in the many applications for which diamond is uniquely suited by virtue of its special physical and electronic properties, by many of the standard reactions of organic chemistry: ##STR1##
The amine groups of the modified diamond can also serve as "anchors" for coupling reagents, spacer arms, etc. For example, amine groups can be reacted with bifunctional molecules such as ICH.sub.2 (CH.sub.2).sub.n C(CH.sub.3).sub.2 --N.sub.3 : EQU R--NH.sub.2 +ICH.sub.2 (CH.sub.2).sub.n C(CH.sub.3).sub.2 --N.sub.3 .fwdarw.R--NH--CCH.sub.2 (CH.sub.2).sub.n C(CH.sub.3).sub.2 --N.sub.3 +HI
The azide functional group can be used in further reaction steps, for example, to immobilize enzymes, antibodies, and other biomolecules on the diamond surface, for service in biosensors, biocatalysts, and other biochemical applications. The azide group may be reduced to an amine under mild conditions, providing amine functional groups that are not subject to the steric constraints of amine groups directly bonded to the diamond surface carbons. Amine-terminated surfaces are suitable for performing protein immobilizations via very mild carbodiimide activation of carboxy groups. For example, very effective coupling reactions can be performed at pH 5.0-6.0 in the presence of low concentrations of the carbodiimide activating agent. As further examples, glycoproteins or glycopeptides are immobilized on a solid phase by (1) opening glycol rings in the sugar chain of the glycoproteins to form aldehyde groups, and (2) binding of the aldehyde groups to the amine groups of a hydrophilic spacer on the surface of the solid phase. Such immobilized biomolecule compositions could take advantage of the properties of diamond and any electronic circuitry formed in or on the diamond substrate.
Successful amination of diamond surfaces has not been reported. Heating or ultraviolet/visible photolysis of ammonia or amines in the presence of hydrogen-terminated diamond does not produce reaction of ammonia or amines with the diamond surface--the diamond surface is simply too inert.
Previous reports had appeared that claimed that thermal ammonia treatment of a chlorinated diamond surface led to an aminated diamond surface. These reports, by the same authors--the first, in Russian (Makal'skii, V. I.; Loktev, V. in Bogatyreva, G. P., ed. Vliyanie Khim. Fiz.-Khim Vozdeistv. Svoistva Almazov (Akad. Nauk Ukr. SSR, Inst. Sverkhtverd. Mater.: Kiev, USSR, 1990) pp. 48-54), the second, in English, ("Surface Modification of Ultradispersed Diamonds," Loktev, V. F.; Makal'skii, V. I.; Stoyanova, I. V.; Kalinkin, A. V.; Likholobov, V. A.; Mit'kin, V. N. Carbon 29 (1991) 817)--reported similar work and claimed that the diamond surface could be aminated by heating a chlorine-terminated diamond surface in the presence of ammonia. These reports were inconsistent with each other and did not present sufficient spectroscopic evidence to support surface amination. The spectroscopic evidence presented in fact was supportive of a hydrogen-terminated diamond surface, rather than an amine-terminated one, confounded by a great deal of contamination by surface hydroxyl and carboxyl moieties. Because these results conflicted with the findings of the inventor, the thermal amination of a chlorine-terminated diamond surfaces was attempted by the methods of the Russian authors, with careful spectroscopic analysis to determine whether amination had actually occurred (see Example 11 below).
Limited photochemical reaction of the gallium arsenide surface has been reported. Monolayer nitridation of the GaAs surface was achieved in ultrahigh vacuum by exposing a half monolayer of NH.sub.3 on the surface (at 102K) to 60 minutes of irradiation with 6.4 eV (193 nm) photons from a pulsed excimer laser (Zhu, X.-Y.; Huett, T.; Wolf, M.; White, J. M. Appl. Phys. Lett. 61 (1992) 3175). This process is a result of nonthermal photodissociation of adsorbed ammonia, and surface --NH.sub.2 is an important intermediate in the nitridation.
This approach, however, requires very vigorous reaction conditions, and does not stop at an amine-terminated surface but proceeds to form a monolayer of GaN. Such an approach is not carried out under mild conditions and does not provide a controllable extent of reaction. Furthermore, the diamond surface is so inert that extremely high energies would be required to break the carbon-hydrogen bonds that terminate the unreacted surface. Such a result has not been reported. A useful method for controlled amination of diamond surfaces must therefore be characterized by a greater selectivity and operability under milder conditions.
Accordingly, it is an object of the present invention to provide a method for controlled chemical modification of diamond surfaces via amination, and methods for further derivatizing or patterning such chemically modified surfaces.
It is another object of the invention to provide such a method which is useful not only for diamond, the most difficult case, but also for related materials from Group IV of the Periodic Table, such as silicon, germanium, germanium carbide, and all polytypes of silicon carbide.
Other objects and advantages of the invention will be apparent from the ensuing disclosure and appended claims.